1. Field of the Invention
This invention is concerned with a pentaerythritol ester base lubricating oil composition for a gas turbine engine. Gas turbine engines are operated under a wide range of temperature conditions. The lubricant must be fluid at extremely low temperatures and at the same time retain its lubricating properties in an engine which produces internal operating temperatures at 450.degree.-550.degree. F. or above. The lubricant is subjected to severe oxidation stresses under the high running temperatures encountered in such engines.
Ester base lubricating oil compositions prepared from pentaerythritol and a mixture of fatty acids and containing selected additive combinations are well known. These lubricants are functional over a wide temperature range and exhibit good thermal and oxidative stability. The search for a still more effective, long lived ester base lubricant composition, however, is a major goal of lubricant manufacturers. In addition, more advanced gas turbine engines currently being developed and tested will put higher stresses on the lubricant composition and are projected to require improved lubricant compositions.
2. Summary of the Invention
The synthetic lubricating oil composition of the invention comprises a major portion of an aliphatic ester base oil containing a phenyl naphthylamine, a dialkyldiphenylamine, a polyhydroxy anthraquinone, a hydrocarbyl phosphate ester and an alkyl thioacid ester. More specifically, the lubricating oil composition of the invention comprises a major portion of an aliphatic ester base oil formed from the reaction of pentaerythritol and an organic monocarboxylic acid having from about 2 to 18 carbon atoms per molecule containing:
(A) FROM ABOUT 0.3 TO 5 PERCENT BY WEIGHT OF THE LUBRICATING OIL COMPOSITION OF PHENYL NAPHTHYLAMINE OR AN ALKYL OR ALKARYL DERIVATIVE OF PHENYL NAPHTHYLAMINES IN WHICH THE ALKYL RADICALS CONTAIN FROM 4 TO 12 CARBON ATOMS,
(B) FROM ABOUT 0.3 TO 5 PERCENT BY WEIGHT OF A DIALKYLDIPHENYLAMINE IN WHICH THE ALKYL RADICALS CONTAIN FROM 4 TO 12 CARBON ATOMS,
(C) FROM ABOUT 0.01 TO 0.5 PERCENT BY WEIGHT OF A POLYHYDROXYANTHRAQUINONE,
(D) FROM ABOUT 0.25 TO 10 PERCENT BY WEIGHT OF A HYDRO CARBYL PHOSPHATE ESTER IN WHICH SAID HYDROCARBYL RADICAL CONTAIN AN ARYL RING AND HAS FROM ABOUT 6 TO 18 CARBON ATOMS, AND
(E) FROM ABOUT 0.01 TO 1.0 PERCENT BY WEIGHT OF A ALKYL THIOACID ESTER AS HEREINAFTER DESCRIBED.
The lubricating oil composition of the invention provides substantial improvements in oxidative stability, particularly excellent control of acidity and viscosity increase under severe oxidizing conditions.